I've got the stickered Silmarillion copy Deagol may be referring to (unless, of course, Deagol has a copy also!).

Its just a big blue sticker (not dark; quite light) with 'A&U' at the top; leaving a space underneath.

No UK bookseller up to ~1995 could vary book prices though (as you state). GA&U was signed up to the Net Book Agreement (some publishers were not), therefore all books carried the 'Net UK Price' blurb regarding book prices.

To add to this oddity, is the information Deagol had passed on to him from a former employee at GA&U, regarding stickers generally. It seems that these were probably illegal too, as they were clearly able to be removed i.e. were not permanent.

All the same, the blank labels (as you have now confirmed a totally different book with one present) do seem a slight puzzle. The most likely explanation is that they were for a market that did not require a UK price i.e. Book Club, or for Export.